Elastomerically impregnated ePTFE to enhance stretch and recovery properties for vascular grafts and coverings

ABSTRACT

An elastomerically recoverable PTFE material is provided including a longitudinally compressed fibrils of ePTFE material penetrated by elastomeric material within the pores defining the elastomeric matrix. The elastomeric matrix and the compressed fibris cooperatively expand and recover without plastic deformation of the ePTFE material. The material may be used for various prosthesis, such as a vascular a prosthesis like a patch, a graft and an implantable tubular stent. Further, a method of producing the elastomerically recoverable PTFE material is provided herein.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional patent applicationSer. No. 10/179,484, filed on Jun. 25, 2002, which is incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to an elastomericallyrecoverable PTFE which is made from expanded, porouspolytetrafluoroethylene (ePTFE) and impregnated with elastomer to belongitudinally compliant for allowing at least a portion of the ePTFEstructure to stretch and recover along the longitudinal axis thereof.

BACKGROUND OF RELATED TECHNOLOGY

It is well known in the art that polymers, such aspolytetrafluoroethylene (PTFE), are used to form a prosthesis. A tubulargraft may be formed by stretching and expanding PTFE into a structurereferred to as expanded polytetrafluoroethylene (ePTFE). Tubes formed ofePTFE exhibit certain beneficial properties as compared with textileprostheses. The expanded PTFE tube has a unique structure defined bynodes interconnected by fibrils. The node and fibril structure definespores that facilitate a desired degree of tissue ingrowth whileremaining substantially fluid-tight. Tubes of ePTFE may be formed to beexceptionally thin and yet exhibit the requisite strength necessary toserve in the repair or replacement of a body lumen. The thinness of theePTFE tube facilitates ease of implantation and deployment with minimaladverse impact on the body.

While exhibiting certain superior attributes, ePTFE material is notwithout certain disadvantages. One disadvantage is the porosity of theePTFE structure which permits cellular ingrowth. The ingrowth isundesirable if one uses the ePTFE material as a temporary graft tobridge vessels and it is desired to have clear access to the ePTFE graftfor replacement or removal.

U.S. Pat. No. 5,665,114 to Weadock et al. discloses an implantableprosthesis made of ePTFE wherein the pores are filled with an in situcross-linkable biocompatible and biodegradable material. Thebio-material may be applied to the ePTFE using force to fill the poreswith a dispersion or solution of the biomaterial, which is subsequentlyinsolubilized therein.

U.S. Pat. No. 5,152,782 to Kowligi et al. discloses a non-porouselastomeric coating on a PTFE graft. The elastomeric coating is made ofpolyurethanes or silicone rubber elastomers. The elastomeric coating isapplied to the graft by radially expanding the PTFE graft, and dippingor spraying the graft with the elastomeric coating. The radial expansionis controlled to ensure that the polymer coating penetration isrestricted to the outer layers of the PTFE tube.

Another disadvantage is the ePTFE material has a tendency to leak bloodat suture holes and often propagate a tear line at the point of entry ofthe suture. The suture holes in ePTFE do not self-seal due to theinelasticity of ePTFE material. As a result, numerous methods oforienting the node and fibril structure have been developed to preventtear propagation. These processes are often complicated and requirespecial machinery and/or materials to achieve this end. Prior artsuggests encapsilling the ePTFE material with a liquid elastomer layer,the elastomer fills in and seals that suture hole.

U.S. Pat. No. 5,192,310 to Herweck et al. discloses a self-sealing PTFEor ePTFE vascular graft having a primary and secondary lumen. Theprimary lumen is to accommodate blood flow. The secondary lumen sharesthe outer wall as a common wall with the primary lumen. The secondarylumen is filled with a non-biodegradable elastomer material, such assilicone rubber, polyurethane, polyethers or fluoropolymers.

U.S. Pat. No. 5,904,967 to Ezaki et al. discloses a puncture resistantbio-compatible medical material for use on as grafts or artificial bloodvessels. The puncture resistant material is sandwiched between twoporous layers of the graft. The porous layers may be made of polyesterresin or polyethylene terephthalate/polybuthylene terephthalate. Thepuncture resistant layer may be a styrene and/or olefin elastomer orisoprene derivatives. The layers are bonded by an adhesive or by fusionwith heat.

Another problem is that ePTFE material exhibits a relatively low degreeof longitudinal compliance. Expanded PTFE is generally regarded as aninelastic material. It has little memory and stretching results indeformation. In those instances when a surgeon will misjudge the lengthof the graft that is required to reach between the selected artery andvein, the surgeon may find that the graft is too short to reach thetargeted site once the graft has been tunneled under the skin. ExpandedPTFE vascular grafts typically exhibit minimal longitudinal compliance,and hence the graft does not stretch significantly along itslongitudinal axis. Accordingly, in such cases, the surgeon must thenremove the tunneled graft from below the skin and repeat the tunnelingprocedure with a longer graft.

Elasticity of an ePTFE vascular graft is important when used for bypassimplants such as an axillofemoral bypass graft, wherein the vasculargraft extends between the femoral artery in the upper leg to theaxillary artery in the shoulder, as well as a femoropopliteal bypassgraft extending below the knee. Such bypass grafts often placerestrictions upon the freedom of movement of the patient in order toavoid pulling the graft loose from its anchor points. For example, inthe case of the axillofemoral bypass graft, sudden or extreme movementsof the arm or shoulder must be entirely avoided. Similarly, in the caseof the femoropopliteal bypass graft, bending the knee can placedangerous stress upon the graft. The above-described restricted movementis due largely to the inability of the ePTFE vascular graft to stretchalong its longitudinal axis when its associated anchor points are pulledapart from one another. Such restrictive movement is especiallyimportant during the early period of healing following implantation whenthere is still little tissue incorporation into the graft and it canmove within the subcutaneous tunnel.

It is desirable to incorporate elastomeric properties into the PTFE.This incorporation is difficult because PTFE is a hydrophobic materialmaking it difficult to wet with the hydro-based elastomers, and theelastomers are hydrophilic making them naturally attracted to otherelastomeric molecules. When elastomeric material is applied to PTFE thetwo materials repel each other and the elastomer flow away from thenodes to a less hydrophobic area, the pores. The pores between thefibrils, are too small for the elastomeric material to penetrate. Thus,the elastomer remains on the surface of the fibrils and coat theexterior of PTFE.

Prior art suggests surface coating the ePTFE material with elastomer bydipping, spraying, or adhesive bonding. One disadvantage is that thecoating may flake or separate from the ePTFE material, as well as add tothe thickness of the ePTFE material.

U.S. Pat. No. 4,304,010 to Mano discloses a tubular prosthesis which ismade of PTFE with a porous elastomeric coating on the outer surface. Theelastomeric coating, which may be cross-linked, is described as beingfluorine rubber, silicone rubber, urethane rubber, acrylic rubber ornatural rubber, and may be applied to the PTFE prosthesis by wrapping,dipping, spraying or use of negative pressure.

U.S. Pat. No. 5,026,591 to Henn et al. discloses a coating product whichcontains a substrate and scaffolding, such as PTFE or ePTFE, where thepores are filled with a thermoplastic or thermosetting resin. Thesubstrate may be of a diverse selection; i.e., woven, non-woven, fabric,paper, or porous polymer. Application of the resin to the PTFE substrateuses rollers to provide a controlled even coating.

U.S. Pat. No. 5,653,747 to Dereume discloses a stent to which a graft isattached. The graft component is produced by extruding polymer insolution into fibers from a spinnerette onto a rotating mandrel. A stentmay be placed over the fibers while on the mandrel and then anadditional layer of fibers spun onto the stent. The layer of layers offibers may be bonded to the stent and/or one another by heat or byadhesives. The porous coating may be made from a polyurethane orpolycarbonate urethane which may be bonded by heat or by adhesion to thesupport.

U.S. Pat. No. 4,321,711 to Mano discloses a vascular prosthesis of PTFEwith an anti-coagulant coating and bonded to its outer surface a porouselastomer coating containing a coagulant. The elastomer is used in itscrosslinked state and is made of fluorine rubber, silicone rubber,urethane rubber, acrylic rubber or natural rubber. The elastomericcoating is bonded to the PTFE by dipping, spraying and/or applyingnegative pressure to inside wall PTFE to pass elastomer through thewall.

U.S. Pat. No. 4,955,899 to Della Conna et al. discloses a longitudinallycompliant PTFE graft. The PTFE tube is longitudinally compressed and theouter wall of the PTFE is coated with a biocompatible material, such aspolyurethanes or silicone rubber elastomers. The coating is applied bycompressing the PTFE tube on a mandrel, and dipping or spraying the PTFEwith the elastomer. The elastomer coating is restricted to the outerlayers of the PTFE tube. The elastomer coated PTFE is dried while in thecompressed state.

Other prior art suggests bonding a separate layer of elastomer to theePTFE material to enhance the elasticity. One disadvantage is the addedthickness of the PTFE. Another disadvantage, as stated above with theelastomer coatings, is the layers will separate over time and can flakeoff the PTFE. Examples of bonding layers of elastomer to PTFE isdiscussed below.

U.S. Pat. No. 4,816,339 to Tu et al. discloses a bio-compatible materialmade from layers of PTFE and hydrophobic PTFE fibers coated with anelastomer mixture. The bio-compatible material disclosed is a PTFElayer, elastomer/PTFE mixed layer, elastomer layer and hydrophilicmonomer fibrous elastomer matrix layer. The elastomer layer is made frompolyurethane. The elastomer is applied to the combined PTFE layer byheating and radially expanding the combined PTFE layers and dipping orspraying the combined PTFE layers with elastomer.

U.S. Pat. No. 5,628,782 to Myers et al. discloses a biocompatible basematerial such as PTFE or ePTFE with an outer deflectably secured outercovering. The preferably outer covering is non-elastic porous film orfibers, preferable PTFE. The outer covering is secured to the base byuse of an adhesive.

U.S. Pat. No. 6,156,064 to Chournard discloses a braided self-expandablestent-graft-membrane. This three layer invention has an interior graftlayer which is braided PET, PCV or PU fibers; a middle layer which isthe stent; and an exterior membrane layer which is a silicone orpolycarbonate methane. The membrane layer is applied to the exterior ofthe stent layer by dipping, by braiding, by spraying or by fusing; whichincludes use of adhesive, solvent bonding or thermal and/or pressurebonding.

It is desirable to provide an ePTFE material that achieves many of theabove-stated benefits without the resultant disadvantages associatedtherewith and disadvantages of similar conventional products. It is alsodesirable to make this elastomerically recoverable PTFE materialavailable to be manufactured in a variety of used such as an implantableprosthesis, patch material, graft, or stent.

SUMMARY OF THE INVENTION

The present invention provides an elastomerically recoverable PTFEmaterial that was made of ePTFE material, defined by nodes and fibrils,and an elastomeric matrix. The fibrils were longitudinally compressed,defining a pore size that is a sufficient size to permit penetration ofan elastomeric material. The elastomeric material penetrated the pores,which defined the elastomeric matrix within the pores. The compressedfibrils and elastomeric matrix cooperatively permitted longitudinalexpansion and elastomeric recovery without plastic deformation of theePTFE material.

The elastomerically recoverable PTFE material is used for a variety ofapplications, such as implantable prosthesis. This includes vascularprosthesis such as patches, grafts or implantable tubular stent graftwith a longitudinally expandable stent.

Another aspect of this invention was to provide a method of producing anelastomerically recoverable PTFE material. The steps to produce thismaterial are discussed below.

First step was to provide the ePTFE material defined by the nodes,fibrils and pores with the required dimensions and specifications toproduce the desired end product. The ePTFE material varied in theirdimensions and specifications such as an ePTFE tube defined by aninternal diameter and an external diameter.

Next, the fibrils were compressed longitudinally, the pore size wassufficiently large enough to permit the elastomeric material topenetrate the pores. The compression step was performed by a variety oftechniques. For example, the ePTFE tube was pulled over a mandrel withan outer diameter approximately the same size as the internal diameterof the ePTFE tube. The entire ePTFE tube or at least a portion of theePTFE tube was compressed along the longitudinal axis of the tube whilethe tube was supported by the mandrel. The ePTFE tube was compresseduniformly along the entire length or any portion thereof of the ePTFEmaterial.

Then the elastomeric material was applied within the pores to provide astructurally integral elastomerically recoverable PTFE material.Elastomer was applied by a variety of techniques, such as dipping,spraying or brushing techniques. The elastomeric material was appliedover the entire longitudinally compressed ePTFE material or to anyportion thereof.

One advantage of this method was that the compression step and theapplication steps were interchangeable to produce an elastomericallyrecoverable PTFE material with various properties, such as differentexpansion ratios. The various properties of the final product wereproduced by performing the compression step prior to, between, and/orafter the application of the elastomeric material.

Finally, the elastomeric material was dried within the pores while thefibrils were still longitudinally compressed which defined theelastomeric matrix. An advantage of this method was that the drying stepwas performed between applications of the elastomeric material or aftercompletion of all the elastomeric material application depending on thedesired end product use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the microstructure of ePTFEmaterial defined by nodes 1, fibrils 2 and pores 3.

FIG. 2 and FIG. 3 are schematic representations of the microstructure oflongitudinally compressed ePTFE material defined by nodes 1,longitudinally compressed fibrils 2 and pores 3.

FIG. 4 is a schematic representation of the microstructure of the ePTFEmaterial of the present invention, defined by nodes 1, longitudinallycompressed fibrils 2 and elastomeric matrix 4 within the pores 3.

FIG. 5 is a schematic representation of the ePTFE materials of thepresent invention formed into an implantable tubular graft 5 defined bythe microstructure having nodes 1, longitudinally compressed fibrils 2and elastomeric matrix 4 within the pores 3.

FIG. 6 is a schematic representation of the ePTFE material of thepresent invention formed into a patch 6 defined by the microstructurehaving nodes 1, longitudinally compressed fibrils 2 and elastomericmatrix 4 within the pores 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The invention described herein provides an elastomerically recoverablePTFE material which has a combination of high stretch and elastomericcompression, high flexibility, and high strength without deformation ofthe material. In this regard it not only exceeds previously availablePTFE products, but is unique among elastomer coated plastic materials.

Expanded PTFE

The precursor for this invention is porous ePTFE which is well known inthe art and is described in detail, for example, in U.S. Pat. Nos.3,953,566 and 3,962,153, which is incorporated herein by reference asshown in FIG. 1. Generally, paste-forming techniques are used to convertthe polymer in paste form to a shaped article which is then expanded,after removing the lubricant, by stretching it in one or moredirections; and while it is held in its stretched condition it is heatedto at least 348° C. after which it is cooled. The porosity that isproduced by the expansion is retained for there is little or nocoalescence or shrinking upon releasing the cooled, final article.

Paste-forming of dispersion polymerized poly(tetrafluoroethylene) iswell known commercially. Extrusions of various cross-sectional shapessuch as tubes, rods and tapes are commonly obtained from a variety oftetrafluoroethylene resins, and other paste-forming operations such ascalendering and molding are practiced commercially. The steps inpaste-forming processes include mixing the resin with a lubricant suchas odorless mineral spirits and carrying out forming steps in which theresin is subjected to shear, thus making the shaped articles cohesive.The lubricant is removed from the extruded shape usually by drying.

The paste-formed, dried, unsintered shapes are expanded by stretchingthem in one or more directions under certain conditions so that theybecome substantially much more porous and stronger. Expansion increasesthe strength of PTFE resin within preferred ranges of rate of stretchingand preferred ranges of temperature. It has been found that techniquesfor increasing the crystallinity, such as annealing at high temperaturesjust below the melt point, improve the performance of the resin in theexpansion process.

The porous microstructure of the ePTFE material is affected by thetemperature and the rate at which it is expanded. The structure consistsof nodes 1 interconnected by very small fibrils 2. In the case ofuniaxial expansion the nodes 1 are elongated, the longer axis of a modebeing oriented perpendicular to the direction of expansion. The fibrils2 which interconnected the nodes 1 are oriented parallel to thedirection of expansion. These fibrils 2 appear to be characteristicallywide and thin in cross-section, the maximum width being equal to about0.1 micron (1000 angstroms) which is the diameter of the crystallineparticles. The minimum width may be one or two molecular diameters or inthe range of 5 or 10 angstroms. The nodes 1 may vary in size from about400 microns to less than a micron, depending on the conditions used inthe expansion. Products which have expanded at high temperatures andhigh rates have a more homogeneous structure, i.e., they have smaller,more closely spaced nodes 1 and these nodes 1 are interconnected with agreater number of fibrils 2.

When the ePTFE material is heated to above the lowest crystallinemelting point of the poly(tetrafluoroethylene), disorder begins to occurin the geometric order of the crystallites and the crystallinitydecreases, with concomitant increase in the amorphous content of thepolymer, typically to 10% or more. These amorphous regions within thecrystalline structure appear to greatly inhibit slippage along thecrystalline axis of the crystallite and appear to lock fibrils andcrystallites so that they resist slippage under stress. Therefore, theheat treatment may be considered an amorphous locking process. Theimportant aspect of amorphous locking is that there be an increase inamorphous content, regardless of the crystallinity of the startingresins. Whatever the explanation, the heat treatment above 348° C.causes a surprising increase in strength, often doubling that of theunheated-treated material.

The preferred thickness of ePTFE material ranges from 0.025 millimeterto 2.0 millimeters; the preferred internodal distance within such ePTFEmaterial ranges from 20 micrometers to 200 micrometers. The longitudinaltensile strength of such ePTFE material is preferably equal to orgreater than 1,500 psi, and the radial tensile strength of such ePTFEmaterial is preferably equal to or greater than 400 psi.

Elastomeric Material

The elastomeric material of this invention was biocompatible elastomersuch as polyurethanes, adhesive solutions and elastomeric adhesivesolutions. Suitable candidates for use as an elastomer typically have aShore hardness rating between 70A and 75D. Most of the above-mentionedelastomers can be chemically or biologically modified to improvebiocompatability; such modified compounds are also candidates for use informing elastomeric material impregnation.

Apart from biocompatability, other requirements of an elastomer to be asuitable candidate for use as elastomeric material impregnation are thatthe elastomer be sufficiently elastic to maintain compressed portions ofePTFE material in the compressed condition when it is not beingstretched. The elastomer should also be sufficiently elastic to effectclosure of suture holes formed by a suture needle. The amount ofelastomeric material needed is the amount to impregnate the ePTFE andprovide the desired elasticity for the end product use, withoutsupersaturating the ePTFE and creating an exterior outer coatings ofelastomer on the ePTFE. Yet another requirement of such elastomers isthat they be easily dissolvable in low boiling point organic solventssuch as tetrahydrofuran, methylene chloride, trichloromethane, dioxane,dimethylformamide, and dimethylacetamide (DMAc) by way of example.Finally, suitable elastomeric materials should lend themselves toapplication by either the dip, brush or spray coating methods describedbelow.

The term elastomeric as used herein refers to a substance having thecharacteristic that it tends to resume an original shape after anydeformation thereto, such as stretching, expanding or compression. Italso refers to a substance which has a non-rigid structure, or flexiblecharacteristics in that it is not brittle, but rather has compliantcharacteristics contributing to its non-rigid nature.

The polycarbonate urethane polymers particularly useful in the presentinvention are more fully described in U.S. Pat. Nos. 5,133,742 and5,229,431 which are incorporated in their entirety herein by reference.These polymers are particularly resistant to degradation in the bodyover time and exhibit exceptional resistance to cracking in vivo. Thesepolymers are segmented polyurethanes which employ a combination of hardand soft segments to achieve their durability, biostability, flexibilityand elastomeric properties.

The polycarbonate urethanes useful in the present invention are preparedfrom the reaction of an aliphatic or aromatic polycarbonate macroglycoland a diisocyanate n the presence of a chain extender. Aliphaticpolycarbonate macroglycols such as polyhexane carbonate macroglycols andaromatic diisocyanates such as methylene diisocyanate are most desireddue to the increased biostability, higher intramolecular bond strength,better heat stability and flex fatigue life, as compared to othermaterials.

The polycarbonate urethanes particularly useful in the present inventionare the reaction products of a macroglycol, a diisocyanate and a chainextender.

A polycarbonate component is characterized by repeating

units, and a general formula for a polycarbonate macroglycol is asfollows:

wherein x is from 2 to 35, y is 0, 1 or 2, R either is cycloaliphatic,aromatic or aliphatic having from about 4 to about 40 carbon atoms or isalkoxy having from about 2 to about 20 carbon atoms, and wherein R′ hasfrom about 2 to about 4 linear carbon atoms with or without additionalpendant carbon groups.

Examples of typical aromatic polycarbonate macroglycols include thosederived from phosgene and bisphenol A or by ester exchange betweenbisphenol A and diphenyl carbonate such as(4,4′-dihydroxy-diphenyl-2,2′-propane) shown below, wherein n is betweenabout 1 and about 12.

Typical aliphatic polycarbonates are formed by reacting cycloaliphaticor aliphatic diols with alkylene carbonates as shown by the generalreaction below:

wherein R is cyclic or linear and has between about 1 and about 40carbon atoms and wherein R¹ is linear and has between about 1 and about4 carbon atoms.

Typical examples of aliphatic polycarbonate diols include the reactionproducts of 1,6-hexanediol with ethylene carbonate, 1,4-butanediol withpropylene carbonate, 1,5-pentanediol with ethylene carbonate,cyclohexanedimethanol with ethylene carbonate and the like and mixturesof above such as diethyleneglycol and cyclohexanedimethanol withethylene carbonate.

When desired, polycarbonates such as these can be copolymerized withcomponents such as hindered polyesters, for example phthalic acid, inorder to form carbonate/ester copolymer macroglycols. Copolymers formedin this manner can be entirely aliphatic, entirely aromatic, or mixedaliphatic and aromatic. The polycarbonate macroglycols typically have amolecular weight of between about 200 and about 4000 Daltons.

Diisocyanate reactants according to this invention have the generalstructure OCN—R′—NCO, wherein R′ is a hydrocarbon that may includearomatic or nonaromatic structures, including aliphatic andcycloaliphatic structures. Exemplary isocyanates include the preferredmethylene diisocyanate (MDI), or 4,4-methylene bisphenyl isocyanate, or4,4′-diphenylmethane diisocyanate and hydrogenated methylenediisocyanate (HMDI). Other exemplary isocyanates include hexamethylenediisocyanate and other toluene diisocyanates such as 2,4-toluenediisocyanate and 2,6-toluene diisocyanate, 4,4′ tolidine diisocyanate,m-phenylene diisocyanate, 4-chloro-1,3-phenylene diisocyanate,4,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate,1,10-decamethylene diisocyanate, 1,4-cyclohexylene diisocyanate,4,4′-methylene bis (cyclohexylisocyanate), 1,4-isophorone diisocyanate,3,3′-dimethyl-4,4′-diphenylmethane diisocyanate,1,5-tetrahydronaphthalene diisocyanate, and mixtures of suchdiisocyanates. Also included among the isocyanates applicable to thisinvention are specialty isocyanates containing sulfonated groups forimproved hemocompatibility and the like.

Suitable chain extenders included in this polymerization of thepolycarbonate urethanes should have a functionality that is equal to orgreater than two. A preferred and well-recognized chain extender is1,4-butanediol. Generally speaking, most diols or diamines are suitable,including the ethylenediols, the propylenediols, ethylenediamine,1,4-butanediamine methylene dianiline heteromolecules such asethanolamine, reaction products of said diisocyanates with water andcombinations of the above.

The polycarbonate urethane polymers according to the present inventionshould be substantially devoid of any significant ether linkages (i.e.,when y is 0, 1 or 2 as represented in the general formula hereinabovefor a polycarbonate macroglycol), and it is believed that ether linkagesshould not be present at levels in excess of impurity or side reactionconcentrations. While not wishing to be bound by any specific theory, itis presently believed that ether linkages account for much of thedegradation that is experienced by polymers not in accordance with thepresent invention due to enzymes that are typically encountered in vivo,or otherwise, attack the ether linkage via oxidation. Live cellsprobably catalyze degradation of polymers containing linkages. Thepolycarbonate urethanes useful in the present invention avoid thisproblem.

Because minimal quantities of ether linkages are unavoidable in thepolycarbonate producing reaction, and because these ether linkages aresuspect in the biodegradation of polyurethanes, the quantity ofmacroglycol should be minimized to thereby reduce the number of etherlinkages in the polycarbonate urethane. In order to maintain the totalnumber of equivalents of hydroxyl terminal groups approximately equal tothe total number of equivalents of isocyanate terminal groups,minimizing the polycarbonate soft segment necessitates proportionallyincreasing the chain extender hard segment in the three componentpolyurethane system. Therefore, the ratio of equivalents of chainextender to macroglycol should be as high as possible. A consequence ofincreasing this ratio (i.e., increasing the amount of chain extenderwith respect to macroglycol) is an increase in hardness of thepolyurethane. Typically, polycarbonate urethanes of hardnesses, measuredon the Shore scale, less than 70A show small amounts of biodegradation.Polycarbonate urethanes of Shore 75A and greater show virtually nobiodegradation.

The ratio of equivalents of chain extender to polycarbonate and theresultant hardness is a complex function that includes the chemicalnature of the components of the urethane system and their relativeproportions. However, in general, the hardness is a function of themolecular weight of both chain extender segment and polycarbonatesegment and the ratio of equivalents thereof. Typically, the4,4′-methylene bisphenyl diisocyanate (MDI) based systems, a1,4-butanediol chain extender of molecular weight 90 and a polycarbonateurethane of molecular weight of approximately 2000 will require a ratioof equivalents of at least about 1.5 to 1 and no greater than about 12to 1 to provide non-biodegrading polymers. Preferably, the ratio shouldbe at least about 2 to 1 and less than about 6 to 1. For a similarsystem using a polycarbonate glycol segment of molecular weight of about1000, the preferred ration should be at least about 1 to 1 and nogreater than about 3 to 1. A polycarbonate glycol having a molecularweight of about 500 would require a ratio in the range of about 1.2 toabout 1.5:1.

The lower range of the preferred ratio of chain extender to macroglycoltypically yields polyurethanes of Shore 80A hardness. The upper range ofratios typically yields polycarbonate urethanes on the order of Shore75D. The preferred elastomeric and biostable polycarbonate urethanes formost medical devices would have a Shore hardness of approximately 85A.

Generally speaking, it is desirable to control somewhat thecross-linking that occurs during polymerization of the polycarbonateurethane polymer. A polymerized molecular weight of between about 80,000and about 200,000 Daltons, for example on the order of about 120,000Daltons (such molecular weights being determined by measurementaccording to the polystyrene standard), is desired so that the resultantpolymer will have a viscosity at a solids content of 43% of betweenabout 900,000 and about 1,800,000 centipoise, typically on the order ofabout 1,000,000 centipoise. Cross-linking can be controlled by avoidingan isocyanate-rich situation. Of course, the general relationshipbetween the isocyanate groups and the total hydroxyl (and/or amine)groups of the reactants should be on the order of approximately 1 to 1.Cross-linking can be controlled by controlling the reaction temperaturesand shading the molar ratios in a direction to be certain that thereactant charge is not isocyanate-rich; alternatively a terminationreactant such as ethanol can be included in order to block excessisocyanate groups which could result in cross-linking which is greaterthan desired.

Concerning the preparation of the polycarbonate urethane polymers, theycan be reacted in a single-stage reactant charge, or they can be reactedin multiple states, preferably in two stages, with or without a catalystand heat. Other components such as antioxidants, extrusion agents andthe like can be included, although typically there would be a tendencyand preference to exclude such additional components when amedical-grade polymer is being prepared.

Additionally, the polycarbonate urethane polymers can be polymerized insuitable solvents, typically polar organic solvents in order to ensure acomplete and homogeneous reaction. Solvents include dimethylacetamide,dimethylformamide, dimethylsulfoxide toluene, xylene, m-pyrrol,tetrahydrofuran, cyclohexanone, 2-pyrrolidone, and the like, orcombinations thereof. These solvents can also be used to delivery thepolymers to the ePTFE layer of the present invention.

A particularly desirable polycarbonate urethane is the reaction productof polyhexamethylenecarbonate diol, with methylene bisphenyldiisocyanate and the chain extender 1,4-butanediol.

The solvents used in the present invention must be capable of wettingthe membrane surface and penetrating the pores. In the case of ePTFEmembranes, wettability of the surface is difficult to accomplish due tothe surface tension properties of the fluoropolymeric structure. Manysolvents will not readily wet the surface of ePTFE sufficiently topenetrate the pores. Thus, the choice of elastomeric material andsolvent must be made with these properties in mind. The elastomericmaterial must be sufficiently dissolvable or softened at the interfaceto flow and penetrate into the membrane pores.

Progressive wetting of the membrane permits the elastomer to enter thepores of the ePTFE material and thus contribute to achieving theadvantages of enhanced stretch and recoverability of the presentinvention. Membranes formed of a hydrophobic material such as ePTFE aredifficult to wet. The type of solvent employed must be both capable ofdissolving the elastomeric material and of wetting the surface of themembrane. Suitable solvent materials, which have been found to be usefulwith polyurethane elastomeric materials and ePTFE membranes include,without limitation, dimethylacetamide, tetrahydrofuran, ethers,methylene chloride, chloroform, toluene and mixtures thereof. Themixture of solvent and elastomeric material provides a balance ofwetting and solvent properties which are particularly effective atcausing penetration and entrapment of the elastomeric material withinthe pores of the ePTFE.

Other solvents may be employed provided they are capable of wetting themembrane, i.e., ePTFE surface, i.e., reducing surface tension such thatthe elastomeric material will flow into the porous microstructure, andare capable of sufficiently dissolving the elastomeric material to causeflow and penetration into the membrane. The solvents chosen should havelittle or no effect on the membrane and serve only as a means toinfiltrate the microstructure and carry the elastomeric materialtherewith. The solvents are then removed by evaporation and theelastomeric material is permitted to dry and resolidify within theporous structure.

Examples of a suitable elastomeric material were sold under thetrademark name “BIONATE” by Polymer Technology Group of Berkley, Calif.and “CORETHANE” by Corvita Corporation of Miami, Fla. Such elastomericmaterials were designed to be dissolved in various solvents for use insolution casting, extruding or for coating of medical products. Thepolycarbonate urethane was dissolved in the solvent known as DMAc.

The method of formulating the liquefied elastomeric material was thesame, as known in the art. This solution was prepared by dissolvingpolyurethane pellets in the above-described DMAc solvent in a heatedglass reactor equipped with a cold water condenser held at 60 C. Suchpolyurethane pellets may also be dissolved in the solvent at roomtemperature through continuous stirring. The use of the heated reactorwas preferred, as it dissolved the polyurethane pellets in a few hours,whereas the method of stirring the solution at room temperature tookapproximately two days.

The preferred solids content for “Corethane 2.5W30” was 7.5% by weight;however, the solids content have ranged up to 15% by weight, dependingupon the specific polymer composition, the dip, brush, spray techniqueparameters, and the intended end uses. Various grades of Corethanesolution are useful depending on the intended end use. Where multipleapplications were employed, the composition of the elastomeric materialwere varied between the applied layers.

Method of Producing Elastomerically Expandable EPTFE Material

In practicing the preferred method, the ePTFE starting material isinitially in the form of a cylindrical tube having an inside diameter upto 50 millimeters. The length may vary depending on the intended enduse.

(i) Longitudinal Compression Process

The compression process of the ePTFE took place either prior to, duringor after the applications of the elastomeric material. The porous wallstructure of the ePTFE easily allowed compression back to the startinglength of the unexpanded PTFE tube structure before it was expandedduring the manufacturing process. The fibrils 2 of the uncompressedePTFE were longitudinally compressed which change the shape of the pore3, and elastomer is permitted to penetrate into the polymeric matrix ofthe ePTFE. Nodes 1 that are forced closer together to allow the fibrils2 to bow out or wrinkle, which may increase the distance between thefibrils 2, and change the pore 3 shape. The pore 3 which permit theelastomer to enter into this space defines the sufficient size of thepore. This sufficient size of the pore 3 is the range between thestarting state and the overly compressed state. The starting state iswhere the node 1 is farthest apart from the opposite node 1 and theconnecting fibrils 2 are taut between the two nodes 1. The overlycompressed state is where the opposite nodes 1 are pushed together,creating two smaller pores 3, and the fibrils 2 are crushed as well. Inthis case the pore 3 space is too small to permit the elastomer to enterwithin this space. One way to compress the ePTFE was to pull the ePTFEtube over a cylindrical supporting mandrel with an outer diameter thatabout equal to the internal diameter of the ePTFE tube. The ePTFE wascompressed along the longitudinal axis of the ePTFE. The compressionprocedure was accomplished by mechanical or thermal procedures. Themechanical procedure included manually squeezing the ePTFE from both ofits ends until a predetermined final compressed length is reached. Thethermal procedure included evenly heating the portion of the ePTFE thatis desired to be compressed. The compression step included uniformlycompressing the PTFE tube along its entire length to produce a tube thatstretched along its entire length up to 90% compression, or localizedcompression to satisfy the intended end use. The percent compression isdefined as the ratio of the final compressed length to the initialuncompressed length. The desired percent compression depends upon theePTFE manufactured expansion ratios, and depending upon the intended useof the final product. Visually the compressed ePTFE material appears tobe denser because the internodal distance has been decreased, as aresult of the nodes 1 being forced closer together. FIGS. 2 and 3 showthe decreased distance between the nodes 1 limits the space availablefor the fibrils 2, resulting in the fibrils 2 crinkling, wrinkling orpossibly folding. Visually the compressed ePTFE material appears to bewrinkled, crinkled or folded, the larger the compression ratio the morewrinkling was seen.

Once the whole or any section of the ePTFE material has been uniformlycompressed along its length, as shown in FIGS. 2 and 3, it is secured onthe mandrel by mechanical means such as Teflon tape or clamps about theends of the compressed ePTFE tube.

(ii) Application of Elastomeric Material Process

Once the pore 3 is a sufficient size to permit penetration of theelastomeric material, the elastomer may be applied. The elastomericmaterial flowed into the pores 3 between the fibrils 2, the point ofleast repelling force, to escape the hydrophobic forces from the ePTFEmaterial, nodes 1 and fibrils 2. The elastomeric material becameentrapped between the pores 3 and embedded the internal fibrils 2 andnodes 1. The embedded fibrils 2 and nodes 1 acted as an internalstructural support for the elastomeric webbing or matrix 4. As mentionedabove, the longitudinal compression process was performed before, duringor after the applications of the elastomeric material depending on thedesired properties for the end-use product. The application processentailed initially dissolving the elastomer in a suitable solution asdiscussed above, defining the elastomeric material. The elastomericmaterial was then applied to the compressed and uncompressed ePTFE byvarious techniques including dip coating, brushing, spraying and thelike

For example, the elastomeric material was applied to the ePTFE by themethod of dip coating which is known in the art by use of a dip coatingmachine. Attention must be placed on the parameters of the machine andlength of the ePTFE to prevent an uneven application. The dip coatingmachine method of application consisted of the mandrel extendedvertically downward from a motor which continuously rotated the mandreland ePTFE material secured thereto (compressed or uncompressed ePTFE).Motor is, in turn, supported by a bracket adapted to travel verticallyupward and downward. Bracket included a smooth bushing through which asmooth vertical support rod passes. Bushing was adapted to slideupwardly and downwardly along support rod. Bracket further included athreaded collar through which a threaded rotatable drive rod passes. Thelowermost end of drive rod is secured to the drive shaft of a secondmotor which rotated in a first rotational direction to raise mandrel andwhich rotated in an opposing rotational direction to lower mandrel. Bothmotor and support rod were supported at their lower ends by a base. Theupper end of support rod was fixedly secured to bracket which rotatablysupported the upper end of drive rod. Motor of dip coating machine wasinitially operated to raise mandrel to its uppermost position. A tall,slender container containing the above-described solution was placedupon base immediately below mandrel. Motor was then operated in thereverse rotational direction to lower mandrel, and ePTFE materialsection secured thereto, into the solution. The variables controlled bydip coating machine include the speed at which mandrel was immersed andwithdrawn and the rotational speed of mandrel. These parameters werecontrolled to ensure that the solution penetrates the ePTFE to allow forthe impregnation of the elastomeric material.

Another example of applying the elastomeric material to the ePTFEmaterial involved the use of spraying which was preformed by a spraycoating machine. The elastomeric material to be sprayed is firstprepared in the same manner as described above for the dip coatingprocess. The elastomeric material was inserted within cylinder of a pumpfor delivery through a plastic tube to a spray nozzle. An inert gas,such as nitrogen, was also supplied to spray nozzle through connectingtube from supply tank. An inert gas was preferably used to minimizereactions which elastomeric material can undergo upon exposure to airand oxygen. The mandrel with the ePTFE was supported for rotation abouta horizontal axis. One end of mandrel was coupled to the drive shaft ofa first motor within motor housing, while the opposite end of mandrelwas rotatably supported by bracket. Both motor housing and bracket weresupported upon the base. The aforementioned first motor continuouslyrotated the mandrel at speeds of up to 500 rotations per minute. Thespray nozzle was supported for reciprocal movement above and alongmandrel. The spray nozzle was secured to support rod which included atits lowermost end a carriage. A threaded drive rod was coupled at afirst end to the drive shaft of a second motor within motor housing forbeing rotated thereby. The opposite end of threaded drive rod wassupported by and freely rotated within bracket. Threaded drive rodthreadedly engaged a threaded collar within carriage. Accordingly,rotation of drive rod caused the carriage, and hence spray nozzle, tomove in the directions designated by dual headed arrow, depending uponthe direction of rotation of drive rod. A pair of micro switches whichwere periodically engaged by carriage and which, when actuated, reversethe direction of rotation of threaded drive rod in a manner which causedspray nozzle to reciprocate back and forth along mandrel. Spray nozzlemade several passes along mandrel, repetitively spraying ePTFE materialas it rotated. Spray nozzle was caused to travel at a linear speed of upto 50 centimeters per minute. The amount of elastomeric materialresulted from this spraying process was determined by the speed ofrotation of mandrel, the linear speed of spray nozzle, the concentrationof the elastomeric material, as well as the rates of delivery of theelastomeric material. These rates of delivery ranged up to 5 millilitersper minute for the solution, and up to 5 liters per minute for thenitrogen gas. The spray application was repeated as needed to reach thedesired properties and amount of elasticity for the end product use.

A final example of applying the elastomeric material to the ePTFEmaterial involved the use of a brushing technique which was preformed bythe same manner as the spraying machine by securing the mandrel andproviding a means of rotation for even application. But instead ofspraying the elastomeric material, it was evenly brushed onto thecompressed or uncompressed ePTFE.

The number of elastomeric material applications ranged between 1 and 100times, depending upon the concentration of the elastomeric material usedin the application process, depending upon the application techniquechosen and parameters of that technique, and depending upon the intendeduse of the end product.

While the application of elastomeric material by dipping, brushing andspraying methods described above were directed to the application ofentire ePTFE material, those skilled in the art will appreciate thatsuch application may be used on only portions of the compressed oruncompressed ePTFE material as well.

(iii) Drying Process

The drying process was performed upon the completion of the applicationof the elastomeric material, or between applications of the elastomericmaterial. The drying process solidified the elastomeric material withinthe pores 3, defining, the elastomeric matrix 4, by evaporating thesolvent and completing the impregnation of the elastomeric materialwithin the pores 3 of the ePTFE as shown in FIG. 4. The drying processdepended on the solvent used, which can include placing the mandrel withthe ePTFE into the oven or allowing the ePTFE to dry at ambientconditions over an extended period of time. The drying processevaporated some of the solvent if used between elastomer applications,or evaporated all of the solvent at the completion of the final product.Once the drying process was complete, the elastomerically recoverableePTFE was removed from the mandrel. While the above describes the dryingprocess and elastomeric application as separate steps, one canappreciate that the steps may be simultaneously occurring. For example,certain solvent concentrations used with the spray technique canevaporate at ambient temperatures upon application of the elastomer.

The elastomer matrix 4 serves two vital purposes; as a bonding agentbasically holding the ePTFE in the compressed state, and as a recoveryagent where after the material is longitudinally stretched the elastomermatrix recovers the material back to the compressed state withoutdeformation.

The use of an elastomerically recoverable ePTFE tube as a vascular graft5 implanted within a patient provided an axillofemoral bypass graft. Thelower end of vascular graft was anchored to femoral artery, while theupper end of vascular graft was anchored to axillary artery. Whenconventional PTFE grafts were used to perform such a bypass, raising ofthe arm placed tension on graft, and placed stress upon the sutured endsof graft, sometimes caused such ends to pull loose from the points atwhich they were anastomosed to the aforementioned arteries. In contrast,the use of elastomerically recoverable ePTFE vascular graft 5 in suchapplications permitted the graft to be stretched without imparting unduestress upon the anchored ends and the graft recovers to the originalsize, thereby permitting the patient greater freedom of movement, morecomfort and no need to replace due to flaking or deformation.

The above-described elastomerically recoverable ePFTE material or patch6 was implanted in the same manner as was currently used to implantconventional ePTFE tubes, patches, grafts, or tubular stent graft, andthe like as shown in FIGS. 5 and 6. Moreover, the elastomeric materialminimized suture hole bleeding at the time of implantation, increasedsuture retention strength, reduced serious weepage, and inhibited tissueingrowth because of its recovery and compression properties. While thisinvention was described with reference to preferred embodiments thereof,the description was for illustrative purposes only and was not to beconstrued as limiting the scope of the invention. Various modificationsand changes may be made by those skilled in the art without departingfrom the true spirit and scope of the invention as defined by theappended claims.

EXAMPLES Example 1

Placed 6 mm diameter ePTFE excel soft tube with 1500% expansion over amandrel of equal diameter. Manually longitudinally compressed the tubeabout 50% and secured the ends of the tube with Teflon tape. While inthe compressed state, applied 7.5% Corethane 2.5W30 in DMAc by brushingtechnique. Placed tube into oven at 110° F. to dry for 10 min. Removefrom oven. Repeated the application of elastomeric material by thebrushing technique 2 more times. Then, removed tube and dried after eachapplication from mandrel. The elastomerically recoverable PTFE tube waslongitudinally stretch up to 90% its original length. Once thestretching force was removed the tube recovered to its original lengthwithout deformation. The stretching and recovery was repeated multipletimes without deforming the elastomerically recoverable PTFE tube.

Example 2

Placed 6 mm diameter ePTFE tube of 800% expansion, expansion velocity of35 cm/sec, over a mandrel with similar diameter. Evenly applied the 7.5%Corethane 2.5W30 in DMAc by the spraying technique. Placed tube intooven at 110° C. for 10 min. and slightly dried the Corethane. Then,removed tube from oven, and repeated elastomeric application by thespray technique and again dried the 2^(nd) application. Then, removedfrom oven and longitudinally compressed ePTFE penetrated withelastomeric material about 50% while on the mandrel, and secured withTeflon tape. A third application of Corethane was applied by the spraytechnique while the tube was in the compressed state. The tube wasplaced in oven for 10 min. The tube was removed from the oven, and thenremoved from the mandrel. The tube longitudinally stretched about 300%.Upon release of stretching force the elastomerically recoverable PTFEtube recovered to original size. The stretching and recovery wasrepeated multiple times without deforming the material.

1. An elastomerically recoverable PTFE material comprising: (a) an ePTFEmaterial defined by nodes and fibrils, said fibrils being inlongitudinally compressed state and defining pores of a size sufficientto permit penetration of an elastomeric material; and (b) an elastomericmatrix within said pores; said compressed fibrils and elastomeric matrixcooperatively permitting longitudinal expansion and elastomeric recoverywithout plastic deformation of said ePTFE material, wherein saidelastomeric matrix is a polycarbonate urethane material having a shorehardness rating between 70A and 75 D.
 2. A vascular prosthesis of claim1 wherein said elastomeric recovery of PTFE matrix is a vascularprosthesis.
 3. The vascular prosthesis of claim 2 wherein said vascularprosthesis is a patch.
 4. The vascular prosthesis of claim 2 wherein thevascular prosthesis is a graft.
 5. An implantable tubular stent graftcomprising: (a) an ePTFE material defined by nodes and fibrils, saidfibrils being in longitudinally compressed state and defining pores of asize sufficient to permit penetration of an elastomeric material; (b) anelastomeric matrix within said pores; said compressed fibrils andelastomeric matrix cooperatively permitting longitudinal expansion andelastomeric recovery without plastic deformation of said ePTFE material,wherein said elastomeric matrix is a polycarbonate urethane materialhaving a shore hardness rating between 70A and 75 D; and (c) alongitudinally expandable stent.
 6. A method of producing anelastomerically recoverable PTFE structure comprising the steps of: (a)providing an ePTFE material defined by nodes, fibrils and pores, whereinsaid pore size is a space defined by the distance between said nodes anddistance between said fibrils; (b) providing an elastomeric material,said elastomeric matrial is a polycarbonate urethane material having ashore hardness rating between 70A and 75 D; (c) compressing said fibrilslongitudinally wherein said pore size is sufficient to permitpenetration of said elastomeric material; and (d) applying saidelastomeric material within said pores to provide a structurallyintegral elastomerically recoverable PTFE material.
 7. The methodaccording to claim 6 further comprising the step of permitting theelastomeric material to dry within said pores while said fibrils arestill longitudinally compressed defining the elastomeric matrix.
 8. Themethod according to claim 6 wherein said ePTFE material is a tube,having an internal diameter and an external diameter.
 9. The methodaccording to claim 8 wherein said compressing step includes the stepsof: (a) pulling the ePTFE tube over a mandrel having an outer diameterof approximately the same dimensions as the internal diameter of theePTFE tube; and (b) compressing at least a portion of the ePTFE tubealong the longitudinal axis of the tube while the tube is supported bythe mandrel.
 10. The method according to claim 6 wherein said applyingstep includes the step of dip coating at least the compressed portion ofthe ePTFE material into a container of elastomeric material.
 11. Themethod according to claim 6 wherein said applying step includes the stepof spray coating at least the compressed portion of the ePTFE materialwith the elastomeric material.
 12. The method according to claim 6wherein said applying step includes the step of brushing the elastomericmaterial onto at least the compressed portion of the ePTFE material. 13.The method according to claim 6 wherein said compressing step includesthe step of compressing the ePTFE material uniformly along its entirelength, and wherein said applying step includes the applying elastomericmaterial over the entire longitudinally compressed ePTFE material.14-21. (canceled)